The present invention relates generally to the field of electrochemical cells. More particularly, this invention pertains to lithium batteries in which the cathode comprises an electroactive sulfur-containing material and the electrolyte comprises reactive components that enhance the capacity of the lithium battery.
Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent specifications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
As the evolution of batteries continues, and particularly as lithium batteries become more widely accepted for a variety of uses, the need for safe, long lasting high energy batteries becomes more important. There has been considerable interest in recent years in developing high energy density cathode-active materials for use in high energy primary and secondary batteries with lithium containing anodes. Various types of cathode materials for the manufacture of lithium batteries are known in the art.
One class of lithium batteries known in the art are rechargeable lithium batteries where the battery is able to undergo multiple discharge and recharge cycles. During discharge of a lithium cell, lithium ions are formed and extracted from the anode and inserted into the cathode. On recharge, the reverse process occurs. The electrodes used in these batteries can have a dramatic effect on the performance of the battery and, in particular, on cycle life.
Another class of lithium batteries known in the art are primary lithium batteries. A primary battery differs from a rechargeable battery in that it is only designed to be discharged once. In fact, because of the design, attempts to recharge a primary battery may create safety problems and may be only partially effective for a very limited number of cycles. Examples of lithium primary cells are described by Nishio et al. in Handbook of Battery Materials, Chapter 2, xe2x80x9cPractical Batteriesxe2x80x9d, pp. 31-40, Elsevier, Amsterdam, (1999) and by Linden in Handbook of Batteries, Chapter 14, pp. 5-6, McGraw-Hill, N.Y. (1995). Primary, non-rechargeable cells, with their single discharge, have a short lifetime and their disposal burden is high, which makes the choice of the cathode material and its impact on the environment of great importance. Sulfur is an attractive cathode-active material for primary cells, both from an environmental perspective and from its very high theoretical specific capacity of 1675 mAh/g in the lithium-sulfur couple.
U.S. Pat. No. 4,410,609 to Peled et al. describes a primary cell comprising an anode consisting of lithium or a dischargeable alloy of lithium, an electrolyte comprising a solvent to dissolve both an electrolyte salt and polysulfides at a low concentration, and an inert porous cathode current collector, which may be loaded with sulfur. Yamin et al., in Electrochemical Society Proceedings, 1984, Volume 84-1, 301-310, describe low rate lithium/sulfur batteries in which the primary cells have a porous carbon cathode current collector impregnated with sulfur and in which the cell""s electrolyte is a lithium polysulfide saturated solution of 1M LiClO4 in tetrahydrofuran-toluene mixtures. The room temperature energy density for these cells is reported to be 730 Wh/Kg.
In a study of dioxolane-based solvents for lithium-sulfur batteries, Peled et al., in J. Electrochem. Soc., 1989, 136, 1621-1625, report that dioxolane-rich solvents are compatible with lithium but that sulfur utilization is only 50% due to the final reduction (discharge) product, Li2S2.
There is a need to enhance the performance of primary and rechargeable lithium electrochemical cells. In studies on lithium/thionyl chloride cells, performance enhancement has been achieved by the addition of halide additives. For example, Linden, in Handbook of Batteries, Chapter 14, pp. 44-47, McGraw-Hill, New York (1995), summarizes data showing an increase in cell voltage and energy density by the addition of BrCl to lithium/thionyl chloride cells. In U.S. Pat. Nos. 4,784,925 and 4,784,927 to Klinedinst et al., small quantities of iodine or iodine monochloride are reported to act as catalysts to increase output voltage and output capacity of lithium/thionyl chloride cells.
Sodium-sulfur cells, which typically operate at high temperatures, such as 300xc2x0 C. to 350xc2x0 C., also typically operate at a capacity less than theoretical to avoid precipitation of insoluble Na2S and Na2S2. U.S. Pat. No. 4,018,969 to Fisher et al., and U.S. Pat. Nos. 4,184,013, 4,216,276, and 4,238,553 to Weddigen et al. describe additives which increase the solubility of Na2S and Na2S2 in the liquid sulfur cathode and thereby increase the capacity of high temperature sodium-sulfur cells.
Despite the various approaches proposed for the fabrication of lithium cells, there remains a need for higher energy density and safer and more environmentally acceptable primary and rechargeable lithium cells.
It is, therefore, an object of the present invention to provide lithium cells which have higher energy density.
It is another object of the present invention to provide cells which are safe and which comprise environmentally acceptable materials.
The present invention pertains to a lithium electrochemical cell comprising: (a) a solid lithium anode; (b) a solid cathode comprising an electroactive sulfur-containing material; and (c) a non-aqueous electrolyte interposed between the solid anode and the solid cathode, which electrolyte comprises: (i) one or more lithium salts; (ii) one or more non-aqueous solvents; and (iii) one or more capacity-enhancing reactive components.
In one embodiment, the one or more capacity-enhancing reactive components comprise an anion receptor. In one embodiment, the one or more capacity-enhancing reactive components comprise an electron transfer mediator.
In one embodiment, the anion receptor comprises a polyalkyleneamine compound of the formula (xe2x80x94N(R)xe2x80x94CH2xe2x80x94CH2xe2x80x94)q, where q is an integer equal to or greater than 2, and R is a substituent selected from the group consisting of CF3SO2, CF3CO, CN, SO2CN, and (xe2x80x94CH2xe2x80x94CH2xe2x80x94N(R1)xe2x80x94)p, where R1 is selected from the group consisting of H, CF3SO2, CF3CO, CN, and SO2CN, and p is an integer from 1 to 4.
In one embodiment, the anion receptor is a boron moiety, BX3, where X, is the same or different at each occurrence and is an electron withdrawing moiety selected from the group consisting of F, perfluoroalkyl, CF2xe2x95x90CFxe2x80x94, pentafluorophenyl, 3,4,5-trifluorophenyl, CF3SO2, N(CF3SO2)2, C(CF3SO2)3, and 
where R2 is the same or different at each occurrence and is selected from the group consisting of H, F, CF3, COCF3, SO2CF3, and SO2F.
In one embodiment, the anion receptor is present in the amount of 0.2% to 25% by weight of the electrolyte. In one embodiment, the anion receptor is present in the amount of 0.5% to 10% by weight of the electrolyte.
In one embodiment, the capacity-enhancing reactive component comprises components of formula: 
wherein:
R4 is the same or different at each occurrence and is selected from the group consisting of H, alkyl, alkenyl, aryl, or substituted derivatives thereof;
E is the same or different at each occurrence and is selected from the group consisting of O, NR5, and S, where R5 is alkyl, aryl, or substituted derivatives thereof;
a is an integer from 0 to 1; and
r is an integer from 2 to 5.
In one embodiment, the electron transfer mediator is of the formula: 
wherein:
Z is the same or different at each occurrence and is selected from the group consisting of O, S, Se, and NR5, where R5 is alkyl, aryl, or substituted derivatives thereof;
R is the same or different at each occurrence, is selected from the group consisting of alkyl, aryl, F, Cl, CF3, CF3SO2, and N(R5)2, where R5 is alkyl, aryl, or substituted derivatives thereof;
Y is xe2x80x94Cxe2x95x90Cxe2x80x94 or 
and
t is an integer from 0 to 4.
In another embodiment, the electron transfer mediator is of the formula: 
wherein:
Z is the same or different at each occurrence and is selected from the group consisting of O, S, Se, and NR5, where R5 is alkyl, aryl, or substituted derivatives thereof;
R is the same or different at each occurrence and is selected from the group consisting of alkyl, aryl, F, Cl, CF3, CF3SO2, and N(R6)2, where R6 is alkyl, aryl, or substituted derivatives thereof;
Y is xe2x80x94Cxe2x95x90Cxe2x80x94 or 
and
u is an integer from 1 to 6.
In one embodiment, the electron transfer mediator is present in the amount of 0.2% to 25% by weight of the electrolyte. In one embodiment, the electron transfer mediator is present in the amount of 0.5% to 10% by weight of the electrolyte.
In one embodiment, the electron transfer mediator has an oxidation-reduction potential less than 2.2 V. In a preferred embodiment, the electron transfer mediator has an oxidation-reduction potential in the range of 1.5 V to about 2.0 V.
In one embodiment, the one or more non-aqueous solvents are selected from the group consisting of ethers, cyclic ethers, polyethers, sulfones, and sulfolanes.
In one embodiment, the one or more lithium salts are selected from the group consisting of LiBr, LiI, LiSCN, LiBF4, LiPF6, LiAsF6, LiSO3CF3, LiN(SO2CF3)2, LiC(SO2CF3)3, (LiSx)zR, and Li2Sx, where x is an integer from 1 to 20, z is an integer from 1 to 3, and R is an organic group.
In one embodiment, the electroactive sulfur-containing material comprises elemental sulfur. In one embodiment, the electroactive sulfur-containing material, in its oxidized state, comprises one or more polysulfide moieties, xe2x80x94Smxe2x80x94, where m is an integer equal to or greater than 3. In one embodiment, the electroactive sulfur-containing material, in its oxidized state, comprises one or more polysulfide moieties, xe2x80x94Smxe2x88x92, where m is an integer equal to or greater than 3. In one embodiment, the electroactive sulfur-containing material, in its oxidized state, comprises one or more polysulfide moieties, xe2x80x94Sm2xe2x88x92, where m is an integer equal to or greater than 3.
In one embodiment, the electroactive sulfur-containing material, in its oxidized state, is of the general formula:
"Brketopenst"C(Sx)"Brketclosest"n
wherein x ranges from greater than 2.5 to about 50, and n is an integer equal to or greater than to 2.
In one embodiment, the electroactive sulfur-containing material, in its oxidized state, comprises one or more of the polysulfur moieties: 
wherein m, the same or different at each occurrence, is an integer and is greater than 2, and y, the same or different at each occurrence, is an integer and is equal to or greater than 1.
In one embodiment, the electroactive sulfur-containing material, in its oxidized state, comprises one or more of the moieties: 
wherein m is the same or different at each occurrence and is greater than 2.
In one embodiment, the electroactive sulfur-containing material is a polymer comprising polymeric segments of the formula; 
wherein:
Q denotes a carbocyclic repeat unit comprising a carbocycle having from 3 to 12 ring carbon atoms;
S denotes a sulfur atom;
m is the number of sulfur atoms in a given polysulfide linkage, is an integer from 3 to 10, and is the same or different at each occurrence;
n denotes the number of crosslinking polysulfide linkages, is an integer from 1 to 20, and is the same or different at each occurrence; and
v is an integer greater than 1.
In one embodiment, the electroactive sulfur-containing material comprises greater than 50% by weight of sulfur. In a preferred embodiment, the electroactive sulfur-containing material comprises greater than 75% by weight of sulfur. In a more preferred embodiment, the electroactive sulfur-containing material comprises greater than 90% by weight of sulfur.
In one embodiment, the lithium anode is selected from the group consisting of lithium metal, lithium-aluminum alloys, lithium-tin alloys, lithium-intercalated carbons, and lithium-intercalated graphites.
In one embodiment, the cell has an energy density of greater than 1000 Wh/Kg. In one embodiment, the cell has an energy density of greater than 1200 Wh/Kg. In one embodiment, the cell has an energy density greater than 1500 Wh/Kg.
In one embodiment of the present invention, the capacity-enhancing reactive components increase the discharge capacity of the first charge-discharge cycle of the cell by greater than 10%. In one embodiment of the present invention, the capacity-enhancing reactive components increase the total discharge capacities of 30 charge-discharge cycles of the cell by greater than 10%. In one embodiment, the capacity-enhancing reactive components increase the total discharge capacities of 30 charge-discharge cycles of the cell by greater than 30%.
In one embodiment, the cell is a secondary electrochemical cell. In one embodiment, the cell is a primary electrochemical cell.
Another aspect of the present invention pertains to a method of making a lithium electrochemical cell comprising the steps of: (a) providing a solid lithium anode; (b) providing a solid cathode comprising an electroactive sulfur-containing material; and (c) interposing a non-aqueous electrolyte between the anode and the cathode, wherein the electrolyte comprises: (i) one or more lithium salts; (ii) one or more non-aqueous solvents; and (iii) one or more capacity-enhancing reactive components, as described herein.
As will be appreciated by one of skill in the art, features of one aspect or embodiment of the invention are also applicable to other aspects or embodiments of the invention.
One aspect of the present invention pertains to electrochemical cells comprising: (a) a solid lithium anode; (b) a solid cathode comprising an electroactive sulfur-containing material; and (c) a non-aqueous electrolyte interposed between the anode and the cathode, wherein the electrolyte comprises: (i) one or more lithium salts; (ii) one or more non-aqueous solvents; and (iii) one or more capacity-enhancing reactive components. In one embodiment, the one or more capacity-enhancing reactive components comprise an anion receptor. In one embodiment, the one or more capacity-enhancing reactive components comprise an electron transfer mediator. The capacity-enhancing reactive components may be non-electroactive or electroactive. The term xe2x80x9cnon-electroactive,xe2x80x9d as used herein, pertains to a cell component material which does not take part in the electrochemical reaction of discharge in the absence of an electroactive sulfur-containing cathode material.
In the present invention, capacity-enhancing reactive components are added to the non-aqueous electrolyte of a lithium/electroactive sulfur-containing material electrochemical cell to increase the electrochemical capacity of the cell. For an electrochemical cell based on lithium and elemental sulfur, the theoretical energy density is 2,600 Wh/Kg. In one example, the capacity limit for a lithium/elemental sulfur cell is about 730 Wh/Kg (calculated on the basis of all cell components, excluding the case) as reported by Yamin et al. in Electrochemical Society Proceedings, 1984, Volume 84-1, 301-310. Typically, the capacity is limited by the insolubility and electronic non-conductivity of Li2S2, a reduction product of the cell, which prevents the complete reduction of S8 to Li2S. It can be seen that approximately 50% of the theoretical capacity is lost if the electrochemical reaction stops at Li2S2. In other words, to extract maximum capacity from the cell, Li2S2 must be reduced to Li2S.
One route by which the capacity-enhancing reactive components of the present invention enhance the capture of the theoretical capacity of the lithium/sulfur-type cells of the present invention is by increasing the solubility of Li2S2. The solubility of Li2S2 may be increased by the use of anion receptors to react or complex with the Li2S2 produced during discharge. Suitable anion receptors include, but are not limited to, the ammonium, amide and Lewis acid materials described by Dietrich in Pure and Applied Chemistry, 1993, 65, 1457-1464, such as polyammonium macrocycles, polyammonium macrobicycles, polyammonium macrotricycles, guanidinium materials, amide functional molecules, and Lewis acid materials. Lee et al. in U.S. Pat. Nos. 5,705,689 and 5,789,585 describe polyalkyleneamine derivatives as anion receptors which increase the ionic conductivity of solutions of lithium salts in non-aqueous solvents, for example, LiCl and LiBr in tetrahydrofuran. Lee et al. in J. Electrochem. Soc., 1998, 145, 2813-2818, describe borate ester anion receptors which increase the ionic conductivity of solutions of lithium salts, for example, lithium trifluoroacetate, LiF, LiCl, and Lil, in dimethoxyethane.
Suitable capacity-enhancing reactive components of the present invention for use as anion receptors include, but are not limited to, linear, branched, and cyclic polyalkyleneamines, of the formula (xe2x80x94N(R)xe2x80x94CH2xe2x80x94CH2xe2x80x94)q, where R is a moiety selected from the group of electron withdrawing groups, such as, CF3SO2, CF3CO, CN, SO2CN, and q is an integer from 1 to 20; or R is (xe2x80x94CH2xe2x80x94CH2xe2x80x94N(R1)xe2x80x94)p, where R1 is H, CF3SO2, CF3CO, CN, and SO2CN, and p is an integer from 1 to 4, as described, for example, by Lee et al. in U.S. Pat. Nos. 5,705,689 and 5,789,585. Further suitable capacity-enhancing reactive components for use as anion receptors include compositions with NCF3, NSO2F, NSO2CF3, and NCF2N functionality, as described, for example, in U.S. Pat. No. 4,216,276 to Weddigen et al.
Suitable Lewis acid anion receptors include, but are not limited to, boron, tin, and silicon moieties. For example, boron moieties, BX3, where X, is the same or different at each occurrence and is an electron withdrawing moiety selected from the group consisting of F, perfluoroalkyl, CF2xe2x95x90CFxe2x80x94, pentafluorophenyl, 3,4,5-trifluorophenyl, CF3SO2, N(CF3SO2)2, C(CF3SO2)3, and 
where R2 is the same or different at each occurrence, and is selected from the group consisting of H, F, CF3, COCF3, SO2CF3, and SO2F.
The amount of the anion receptor may vary over a wide range from about 0.2% by weight of the total electrolyte components to about 25% by weight. Preferably the amount is from about 0.5% by weight to about 10% by weight. More preferably the amount is from about 1% by weight to about 5% by weight.
Although any of the suitable anion receptors may be used, in each situation the most effective choice will depend on the particular electrolyte solvent system and electrolyte salt.
An alternative route to enhance the capture of the theoretical capacity of the lithium/sulfur-type cells of the present invention is by the addition of an electron transfer mediator to the electrolyte for the reduction of Li2S2 to Li2S. Effective mediators are those whose oxidation-reduction potential is below 2.2 V. Most preferably, the mediators have an oxidation-reduction potential in the range of 1.5 V to 2.0 V.
While not wishing to be bound by any theory, the electron transfer mediators may function by accepting a pair of electrons in an electrochemical reduction process at a voltage in the range of 1.5 V to 2.0 V, and in turn transferring the electrons to Li2S2 with the formation of lithium sulfide, Li2S, liberation of sulfide ion, S2xe2x88x92, and regeneration of the mediator, M. The process may described as:
M+2exe2x88x92xe2x86x92M2xe2x88x92
M2xe2x88x92+Li2S2xe2x86x92Li2S+S2xe2x88x92+M
Alternatively, the electron transfer mediators may function by accepting a single electron in an electrochemical reduction process at a voltage in the range of 1.5 V to 2.0 V and in turn transferring an electron to Li2S2. In a subsequent step, a second electron transfer may take place completing the reduction of Li2S2 to Li2S+S2xe2x88x92. The mediator remains unchanged in this cycle. This process may be described as:
M+exe2x88x92xe2x86x92Mxe2x80xa2xe2x88x92
Mxe2x80xa2xe2x88x92+Li2S2xe2x86x92Li2S2xe2x80xa2xe2x88x92+M
Li2S2xe2x80xa2xe2x88x92+Mxe2x80xa2xe2x88x92xe2x86x92Li2S2+S2xe2x88x92+M
where M represents an electron transfer mediator.
Suitable electron transfer mediators include, but are not limited to, those of formula I: 
wherein:
R4 is the same or different at each occurrence and is selected from the group consisting of H, alkyl, alkenyl, aryl, or substituted derivatives thereof;
E is the same or different at each occurrence and is selected from the group consisting of O, NR5, and S, where R5 is alkyl, aryl, or substituted derivatives thereof;
a is an integer from 0 to 1; and
r is an integer from 2 to 5.
Suitable electron transfer mediators of formula I include, but are not limited to, bis(methoxymethyl)disulfide, bis(methoxymethyl)trisulfide, bis(methoxymethyl)polysulfide, bis(methoxyethyl)disulfide, bis(methoxyethyl)trisulfide, bis(methoxyethyl)polysulfide, bis(3-allyloxy-2-hydroxypropyl)disulfide, bis(3-allyloxy-2-hydroxypropyl)trisulfide, bis(3-allyloxy-2-hydroxypropyl)tetrasulfide, bis(vinyloxyethoxy-2-hydroxypropyl)disulfide, bis(vinyloxyethoxy-2-hydroxypropyl)trisulfide, bis(vinyloxyethoxy-2-hydroxypropyl)polysulfide, bis(2-hydroxy-2-phenylethyl)disulfide, bis(2-hydroxy-2-phenylethyl)trisulfide, bis(2-hydroxy-2-phenylethyl)polysulfide, bis(N,N-dimethylaminoethyl)disulfide, bis(N,N-diethylaminoethyl)disulfide, bis(N,N-dimethylaminoethyl)trisulfide, bis(N,N-diethylaminoethyl)trisulfide, and bis(N,N-dimethylaminoethyl)polysulfide.
Suitable electron transfer mediators also include, but are not limited to, those of formula II: 
wherein:
Z is the same or different at each occurrence and is selected from the group consisting of O, S, Se, and NR5, where R5 is alkyl, aryl, or substituted derivatives thereof;
R is same or different at each occurrence and is selected from the group consisting of alkyl, aryl, F, Cl, CF3, CF3SO2, and N(R5)2, where R5 is alkyl, aryl, or substituted derivatives thereof and is the same or different at each occurrence;
Y isxe2x80x94Cxe2x95x90Cxe2x80x94 or 
and
t is an integer from 0 to 4.
Suitable electron transfer mediators also include, but are not limited to, those of formula III: 
wherein:
Z is the same or different at each occurrence and is selected from the group consisting of O, S, Se, and NR5, where R5 is alkyl, aryl, or substituted derivatives thereof;
R is the same or different at each occurrence and is selected from the group consisting of alkyl, aryl, F, Cl, CF3, CF3SO2, and N(R5)2, where R5 is alkyl, aryl, or substituted derivatives thereof and is the same or different at each occurrence;
Y is xe2x80x94Cxe2x95x90Cxe2x80x94 or 
and
u is an integer from 1 to 6.
Other examples of suitable electron transfer mediators include transition metal complexes, including but not limited to, complexes of phthalocyanines and porphyrins with transition metals, including but not limited to, iron, cobalt, manganese, vanadium, copper, chromium, and nickel, where the complexes are soluble in the electrolyte.
Other examples of suitable electron transfer mediators include bis-pyridinium salts, also known as viologens, and polynuclear aromatic hydrocarbons which can form radical anions upon reduction, where the bis-pyridinium salts and the polynuclear aromatic hydrocarbons are soluble in the electrolyte.
Although any of the suitable electron transfer mediators may be used, in each situation the most effective choice will depend on the solubility and compatibility of the electron transfer mediator with the particular electrolyte solvent system, the electrolyte salt, and the reduction products of the sulfur-containing cathode material.
The amount of the electron transfer mediator may vary over a wide range from 0.2% by weight of the total electrolyte components to about 25% by weight. Preferably the amount is from about 0.5% by weight to about 10% by weight. More preferably the amount is from about 1% by weight to about 5% by weight
In another embodiment of the present invention, an electron transfer mediator may be formed in the electrochemical cell from the reaction of reduction products of the sulfur-containing cathode materials with a precursor moiety. Suitable precursor moieties will possess high reactivity toward cathode reduction products such as, for example, polysulfide anions but be unreactive or possess low reactivity toward electrolyte components. Examples of suitable precursor moieties include, but are not limited to, haloalkyl ethers, haloalkyl amines, alkyl epoxides, aryl epoxides, and glycidyl ethers. Haloalkyl ethers include chloromethyl methyl ether, chloromethyl ethyl ether, chloromethyl benzyl ether, chloroethyl methyl ether, bis(chloromethyl) ether, and the corresponding bromo-derivatives. Haloalkyl amines include N,N-dimethyl-2-chloroethylamine, N,N-diethyl-2-chloroethylamine, and N,N-dimethyl-2-bromoethylamine. Epoxides include propylene oxide, butylene oxide, and styrene oxide. Glycidyl ethers include methyl glycidyl ether, ethyl glycidyl ether, and ethylene glycol methyl glycidyl ether.
The capacity-enhancing reactive components of the present invention are added to the electrolyte, which comprises one or more non-aqueous solvents and one or more ionic electrolyte salts.
In one embodiment of the present invention, the energy density of the cell is greater than 1000 Wh/Kg. In another embodiment, the energy density of the cell is greater than 1200 Wh/Kg. In a preferred embodiment, the energy density of the cell is greater than 1500 Wh/Kg. The term xe2x80x9cenergy density,xe2x80x9d as used herein, relates to cell energy based on the sum of the weights of the anode active components, the cathode active components, and the capacity-enhancing reactive components.
In one embodiment of the present invention, the capacity-enhancing reactive components increase the discharge capacity of the first charge-discharge cycle of the cell by greater than 10%. In one embodiment of the present invention, the capacity-enhancing reactive components increase the total discharge capacities of 30 charge-discharge cycles of the cell by greater than 10%. In a preferred embodiment of the present invention, the capacity-enhancing reactive components increase the total discharge capacities of 30 charge-discharge.cycles of the cell by greater than 30%.
The electrolytes of the electrochemical cells of the present invention comprising one or more capacity-enhancing reactive components may additionally comprise voltage-enhancing reactive components, for example as described in co-pending U.S. patent application Ser. No. 09/709,778, now abandoned, and published as International Publication No. WO 01/35475, entitled xe2x80x9cLithium Primary Batteriesxe2x80x9d to Mikhaylik et al. of the common assignee, both filed on even date herewith.
The term xe2x80x9celectroactive sulfur-containing material,xe2x80x9d as used herein, relates to cathode active materials which comprise the element sulfur in any form, wherein the electrochemical activity involves the breaking or forming of sulfur-sulfur covalent bonds.
Examples of suitable electroactive sulfur-containing materials, include, but are not limited to, elemental sulfur and organic materials comprising both sulfur atoms and carbon atoms, which may or may not be polymeric. Suitable organic materials include those further comprising heteroatoms, conductive polymer segments, composites, and conductive polymers.
In one embodiment, the electroactive sulfur-containing material comprises elemental sulfur. In one embodiment, the electroactive sulfur-containing material comprises a mixture of elemental sulfur and a sulfur-containing polymer.
In one embodiment, the sulfur-containing material, in its oxidized state, comprises a polysulfide moiety, Sm, selected from the group consisting of covalent xe2x80x94Smxe2x80x94 moieties, ionic xe2x80x94Smxe2x88x92 moieties, and ionic Sm2xe2x88x92 moieties, wherein m is an integer equal to or greater than 3, such as for example, elemental sulfur and sulfur-containing polymers. In one embodiment, m of the polysulfide moiety, Sm, is an integer equal to or greater than 6. In one embodiment, m of the polysulfide moiety, Sm, is an integer equal to or greater than 8. In one embodiment, the sulfur-containing material is a sulfur-containing polymer. In one embodiment, the sulfur-containing polymer has a polymer backbone chain and the polysulfide moiety, Sm, is covalently bonded by one or both of its terminal sulfur atoms as a side group to the polymer backbone chain. In one embodiment, the sulfur-containing polymer has a polymer backbone chain and the polysulfide moiety, Sm, is incorporated into the polymer backbone chain by covalent bonding of the terminal sulfur atoms of the polysulfide moiety.
The specific capacity (mAh/g or Ah/Kg) or energy density (Wh/Kg) in electroactive sulfur-containing materials is directly related to the number of electrons participating in the reduction/oxidation (discharge/charge) process. For a disulfide group, (Rxe2x80x2-S-S-Rxe2x80x3, where Rxe2x80x2 and Rxe2x80x3 are independently an organic group), two electrons participate in the discharge/charge process. For higher polysulfides, two electrons participate in each sulfur-sulfur bond reduction. It can be readily appreciated that increased energy densities are obtained in higher polysulfides compared with disulfides.
In one embodiment, the electroactive sulfur-containing material comprises greater than 50% by weight of sulfur. In a preferred embodiment, the electroactive sulfur-containing material comprises greater than 75% by weight of sulfur. In a more preferred embodiment, the electroactive sulfur-containing material comprises greater than 90% by weight of sulfur.
The nature of the electroactive sulfur-containing materials useful in the practice of this invention may vary widely. Further examples of suitable electroactive sulfur-containing materials include, but are not limited to:
(a) an electroactive polycarbon-sulfide material, which in its oxidized state, is of the general formula:
"Parenclosest"C(Sx)"Brketopenst"n
wherein x ranges from greater than 2.5 to about 50, and n is an integer equal to or greater than 2, as described in U.S. Pat. Nos. 5,601,947 and 5,690,702 to Skotheim et al.; and which, in its oxidized state, may comprise one or more of the polysulfur moieties: 
wherein m, the same or different at each occurrence, is an integer and is greater than 2, and y, the same or different at each occurrence, is an integer and is equal to or greater than 1;
(b) an electroactive polyacetylene co-polysulfur material, which, in its oxidized state, is of the general formula: 
xe2x80x83wherein x ranges from greater than 1 to about 100, and n is an integer equal to or greater than 2, as described in U.S. Pat. Nos. 5,529,860 and 6,117,590 to Skotheim et al., and which, in its oxidized state, may comprise one or more of the moieties: 
xe2x80x83wherein m, the same or different at each occurrence, is greater than 2; and
(c) an electroactive, highly crosslinked organic polymer, which polymer comprises polymeric segments of the formula; 
wherein:
Q denotes a carbocyclic repeat unit comprising a carbocycle having from 3 to 12 ring carbon atoms;
S denotes a sulfur atom;
m is the number of sulfur atoms in a given polysulfide linkage, is an integer from 3 to 10, and is the same or different at each occurrence;
n denotes the number of crosslinking polysulfide linkages, is an integer from 1 to 20, and is the same or different at each occurrence; and
v is an integer greater than 1; as described in U.S. patent application Ser. No. 08/995,122, now U.S. Pat. No. 6,201,100, to Gorkovenko et al. of the common assignee and PCT Publication No. WO 99/33130.
Other suitable electroactive sulfur-containing materials comprising polysulfide linkages include, but are not limited to, those described in U.S. Pat. No. 4,664,991 to Perichaud et al. and in U.S. Pat. Nos. 5,723,230, 5,783,330, 5,792,575 and 5,882,819 to Naoi et al.
Other examples of suitable electroactive sulfur-containing polymers include organo-sulfur materials comprising disulfide linkages, although their low specific capacity compared to the corresponding materials comprising polysulfide linkages makes it highly difficult to achieve the high capacities desired for practical electrochemical cells. However, they may be utilized in a blend with elemental sulfur and/or with sulfur-containing polymers comprising a polysulfide moiety in the cathodes of this invention, and may contribute by their electrochemical properties, by their interaction with lithium polysulfides and lithium sulfides generated during the cycling of the cells, and, optionally, by their melting properties during cell fabrication, to achieve the desired high capacities in the electrochemical cells or batteries of the present invention. Examples of electroactive sulfur-containing materials comprising disulfide groups include those described in U.S. Pat. No. 4,739,018 to Armand et al.; U.S. Pat. Nos. 4,833,048 and 4,917,974, both to De Jonghe et al.; U.S. Pat. Nos. 5,162,175 and 5,516,598, both to Visco et al.; and U.S. Pat. No. 5,324,599 to Oyama et al.
The cathodes of the lithium cells of the present invention may further comprise one or more conductive fillers to provide enhanced electronic conductivity. Examples of conductive fillers include, but are not limited to, those selected from the group consisting of conductive carbons, graphites, activated carbon fibers, non-activated carbon nanofibers, metal flakes, metal powders, metal fibers, carbon fabrics, metal mesh, and electrically conductive polymers. The amount of conductive filler, if present, is preferably in the range of 2 to 30% by weight. The cathodes of the present invention may also further comprise other additives such as, for example, metal oxides, aluminas, silicas, and transition metal chalcogenides.
The cathodes of the lithium cells of the present invention may also comprise a binder. The choice of binder material may vary widely so long as it is inert with respect to the other materials in the cathode. Useful binders are those materials, usually polymeric, that allow for ease of processing of battery electrode composites and are generally known to those skilled in the art of electrode fabrication. Examples of useful binders include, but are not limited to, those selected from the group consisting of polytetrafluoroethylenes (Teflon(copyright)), polyvinylidene fluorides (PVF2 or PVDF), ethylene-propylene-diene (EPDM) rubbers, polyethylene oxides (PEO), UV curable acrylates, UV curable methacrylates, and heat curable divinyl ethers, and the like. The amount of binder, if present, is preferably in the range of 2 to 30% by weight.
The cathodes of the lithium cells of the present invention may further comprise a current collector as known in the art. Current collectors are useful in efficiently collecting the electrical current generated throughout the cathode and in providing an efficient surface for attachment of the electrical contacts leading to the external circuit as well as functioning as a support for the cathode. Examples of useful current collectors include, but are not limited to, those selected from the group consisting of metallized plastic films, metal foils, metal grids, expanded metal grids, metal mesh, metal wool, woven carbon fabric, woven carbon mesh, non-woven carbon mesh, and carbon felt.
Cathodes of the lithium cells of the present invention may be prepared by a variety of methods. For example, one suitable method comprises the steps of: (a) dispersing or suspending in a liquid medium the electroactive sulfur-containing material, as described herein; (b) optionally adding to the mixture of step (a) a conductive filler, binder, or other cathode additives; (c) mixing the composition resultingfrom step (b) to disperse the electroactive sulfur-containing material; (d) casting the composition resulting from step (c) onto a suitable substrate; and (e) removing some or all of the liquid from the composition resulting from step (d) to provide the cathode.
Examples of suitable liquid media for the preparation of cathodes of the present invention include aqueous liquids, non-aqueous liquids, and mixtures thereof. Especially preferred liquids are non-aqueous liquids such as, for example, methanol, ethanol, isopropanol, propanol, butanol, tetrahydrofuran, dimethoxyethane, acetone, toluene, xylene, acetonitrile, and cyclohexane.
Mixing of the various components can be accomplished using any of a variety of methods known in the art so long as the desired dissolution or dispersion of the components is obtained. Suitable methods of mixing include, but are not limited to, mechanical agitation, grinding, ultrasonication, ball milling, sand milling, and impingement milling.
The formulated dispersions can be applied to substrates by any of a variety of coating methods known in the art and then dried using techniques, known in the art, to form the solid cathodes of the lithium cells of this invention. Suitable hand coating techniques include, but are not limited to, the use of a wire-wound coating rod or gap coating bar. Suitable machine coating methods include, but are not limited to, the use of roller coating, gravure coating, slot extrusion coating, curtain coating, and bead coating. Removal of some or all of the liquid from the mixture can be accomplished by any of a variety of means known in the art. Examples of suitable methods for the removal of liquids from the mixture include, but are not limited to, hot air convection, heat, infrared radiation, flowing gases, vacuum, reduced pressure, and by simply air drying.
The method of preparing the cathodes of the present invention may further comprise heating the electroactive sulfur-containing material to a temperature above its melting point and then resolidifying the melted electroactive sulfur-containing material to form a cathode active layer having redistributed sulfur-containing material of higher volumetric density than before the melting process.
The electrolytes used in electrochemical or battery cells function as a medium for the storage and transport of ions, and in the special case of solid electrolytes and gel electrolytes, these materials may additionally function as a separator between the anode and the cathode. Any liquid, solid, or gel material capable of storing and transporting ions may be used, so long as the material is electrochemically and chemically unreactive with respect to the anode and the cathode, and the material facilitates the transport of lithium ions between the anode and the cathode. The electrolyte must also be electronically non-conductive to prevent short circuiting between the anode and the cathode.
Typically, the electrolyte comprises one or more ionic electrolyte salts to provide ionic conductivity and one or more non-aqueous liquid electrolyte solvents, gel polymer materials, or polymer materials. Suitable non-aqueous electrolytes for use in the present invention include, but are not limited to, organic electrolytes comprising one or more materials selected from the group consisting of liquid electrolytes, gel polymer electrolytes, and solid polymer electrolytes. Examples of non-aqueous electrolytes for lithium batteries are described by Dominey in Lithium Batteries, New Materials, Developments and Perspectives, Chapter 4, pp. 137-165, Elsevier, Amsterdam (1994). Examples of gel polymer electrolytes and solid polymer electrolytes are described by Alamgir et al. in Lithium Batteries, New Materials, Developments and Perspectives, Chapter 3, pp. 93-136, Elsevier, Amsterdam (1994).
Examples of useful non-aqueous liquid electrolyte solvents include, but are not limited to, non-aqueous organic solvents, such as, for example, N-methyl acetamide, acetonitrile, acetals, ketals, sulfones, sulfolanes, aliphatic ethers, cyclic ethers, glymes, polyethers, phosphate esters, siloxanes, dioxolanes, N-alkylpyrrolidones, substituted forms of the foregoing, and blends thereof. Fluorinated derivatives of the foregoing are also useful.
Examples of ethers include, but are not limited to, dimethyl ether, diethyl ether, methylethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dimethoxymethane, trimethoxymethane, dimethoxyethane, diethoxyethane, 1,3-dimethoxypropane, tetrahydrofuran, tetrahydropyran, 1,4-dioxane, and 1,3-dioxolane.
Examples of polyethers include, but are not limited to, diethylene glycol dimethyl ether (diglyme), triethylene glycol dimethyl ether (triglyme), higher glymes, diethylene glycol divinylether, and triethylene glycol divinylether.
Examples of sulfones include, but are not limited to, sulfolane, 3-methyl sulfolane, 3-sulfolene, and non-symmetrical, non-cyclic sulfones, and fluorinated or partially fluorinated derivatives of the foregoing.
The specific choice of solvent will depend on several factors including self discharge. The term xe2x80x9cself discharge,xe2x80x9d as used herein, relates to the loss of capacity, or charge, when no external load is applied to the cell. An electrolyte comprising one or more non-aqueous electrolyte solvents and one or more electrolyte salts typically interacts with the lithium anode surface to form a solid electrolyte interface (SEI). The SEI allows passage of lithium ions as the cell discharges and at the same time it is desirable that the SEI protects the lithium surface from further reactions with electrolyte, cathode discharge products, or other soluble components of the cathode. In cells comprising electroactive sulfur-containing materials, the SEI should protect the lithium from self discharge, for example, from reaction with possible cathode discharge products such as sulfide ions, polysulfide ions, and other sulfur containing ions, and soluble cathode components such as sulfur. Preferred electrolyte solvents are those which provide low self discharge rates.
These liquid electrolyte solvents are themselves useful as plasticizers for gel polymer electrolytes. Examples of useful gel polymer electrolytes include, but are not limited to, those comprising one or more polymers selected from the group consisting of polyethylene oxides, polypropylene oxides, polyacrylonitriles, polysiloxanes, polyimides, polyphosphazenes, polyethers, sulfonated polyimides, perfluorinated membranes (NAFION(trademark) resins), polydivinyl polyethylene glycols, polyethylene glycol diacrylates, polyethylene glycol dimethacrylates, derivatives of the foregoing, copolymers of the foregoing, crosslinked and network structures of the foregoing, and blends of the foregoing, and optionally plasticizers.
Examples of useful solid polymer electrolytes include, but are not limited to, those comprising one or more polymers selected from the group consisting of polyethers, polyethylene oxides, polypropylene oxides, polyimides, polyphosphazenes, polyacrylonitriles, polysiloxanes, derivatives of the foregoing, copolymers of the foregoing, crosslinked and network structures of the foregoing, and blends of the foregoing.
In addition to solvents, gelling agents, and polymers as known in the art for forming non-aqueous electrolytes, the non-aqueous electrolyte may further comprise one or more ionic electrolyte salts, also as known in the art, to increase the ionic conductivity.
Examples of ionic electrolyte salts for use in the present invention include, but are not limited to, LiSCN, LiBr, LiI, LiClO4, LiAsF6, LiSO3CF3, LiSO3CH3, LiBF4, LiB(Ph)4, LiPF6, LiC(SO2CF3)3, and LiN(SO2CF3)2. Other electrolyte salts useful in the practice of this invention include lithium polysulfides (Li2Sx), and lithium salts of organic ionic polysulfides (LiSxR)n, where x is an integer from 1 to 20, n is an integer from 1 to 3, and R is an organic group, and those disclosed in U.S. Pat. No. 5,538,812 to Lee et al. The lithium polysulfides, Li2Sx, may be formed in situ in Li/S cells by self-discharge of the cells or during the discharge of the cells. Preferred ionic electrolyte salts are LiBr, LiI, LiSCN, LiBF4, LiPF6, LiAsF6, LiSO3CF3, LiN(SO2CF3)2, and LiC(SO2CF3)3.
The electrochemical cells of the present invention may further comprise a separator interposed between the cathode and anode. Typically, the separator is a solid non-conductive or insulative material which separates or insulates the anode and the cathode from each other preventing short circuiting, and which permits the transport of ions between the anode and the cathode.
The pores of the separator may be partially or substantially filled with electrolyte. Separators may be supplied as porous free standing films which are interleaved with the anodes and the cathodes during the fabrication of cells. Alternatively, the porous separator layer may be applied directly to the surface of one of the electrodes, for example, as described in PCT Publication No. WO 99/33125 to Carlson et al. and in U.S. Pat. No. 5,194,341 to Bagley el al.
A variety of separator materials are known in the art. Suitable solid porous separator materials include, but are not limited to, polyolefins, such as, for example, polyethylenes and polypropylenes, glass fiber filter papers, and ceramic materials. Further examples of separators and separator materials suitable for use in this invention are those comprising a microporous pseudo-boehmite layer, which may be provided either as a free standing film or by a direct coating application on one of the electrodes, as described in U.S. patent application Ser. Nos. 08/995,089, now U.S. Pat. Nos. 6,153,337, and 09/215,112, now U.S. Pat. No. 6,306,545, by Carlson el al. of the common assignee. Solid electrolytes and gel electrolytes may also function as a separator in addition to their electrolyte function.
In one embodiment, the solid porous separator is a porous polyolefin separator. In one embodiment, the solid porous separator comprises a microporous pseudo-boehmite layer.
Cells and batteries of the present invention may be made in primary and secondary types and in a variety of sizes and configurations which are known to those skilled in the art. These battery design configurations include, but are not limited to, planar, prismatic, jelly roll, w-fold, stacked, and the like. Although the methods of the present invention are particularly suitable for use with thin film electrodes, they may nevertheless be beneficial in thick film designs.
It is generally accepted that, when low electric currents are desired, the electrodes within the cell should have as much mass and as little surface area as possible. At the expense of power density, this provides for increased energy density while low electrode surface area minimizes undesirable self-discharge reactions. Conversely, when larger electrical discharge currents are required, electrode surface area and power density are maximized at the expense of energy density and self-discharge rate. Thin film electrodes provide high surface area and thereby high power density. Thin film electrodes may be incorporated into a number of battery design configurations, such as prismatic, jelly roll, w-fold and stacked configurations. Alternatively, designs incorporating both low and high surface area regions, as described in U.S. Pat. Nos. 5,935,724 and 5,935,728 to Spillman et al., may be incorporated into jelly roll and other configurations.
Thin film electrodes, in particular, may be configured into prismatic designs. With the drive to conserve weight, thin film barrier materials are particularly preferred, e.g., foils. For example, PCT Publication No. WO 00/36678 (International Appl. NO. PCT/US99/30133) to Thibault et al. describes methods for preparing prismatic cells in which suitable barrier materials for sealed casings, methods of filling cells with electrolyte, and methods of sealing the casing, are described.